1. Field of the Invention
This invention relates to a fatigue-resistant welded joint and to a method for making the same.
2. Description of the Prior Art
Various ways of modifying a primary structural weld, such as a fillet weld, have been used to the improve weld strength, to temper the weld, to reinforce it, or to render it impermeable. Thus, one or more weld metal beads have been superimposed on a structural weld, sometimes to cover it completely and, often, the immediately adjacent portions of at least one of the base metal pieces also. Superimposition of welds is characteristic of multiple pass welding wherein one makes a root pass followed by one or more cover passes, uses a technique such as that of U.S. Pat. No. 1,554,546, uses a Temper Bead Technique (TBT); or welds according to the specification NAVSHIPS 0900-006-9010 Standard for Fabrication Welding, and Inspection of HY-80 Submarine Hulls.
Also, it has been known that the toe of the primary structural weld can be subject to reprocessing with an electric arc without any filler metal, thereby acting to remelt the weld surface and perhaps form a tiny bead therefrom at the toe (as in U.S. Pat. No. 1,688,360 or by so-called Tungsten Inert Gas--i.e., TIG--Dressing). Additionally, covering and bridging passes over primary structural welds have been used for reducing a stress corrosion tendency in nuclear reactor water lines (U.S. Pat. No. 4,049,186).
For the purposes of this application a "toe" of a structural weld is defined by a line where the length of the structural weld metal meets the base metal. Thus, a single pass fillet weld between two angularly abutting pieces of base metal has two toes; so has a single pass seam weld between two pieces of base metal, such seam weld being where two base metal pieces overlap to be joined from the end of one piece to a side of the other; the seam weld is in effect, a form of a fillet weld.
Additionally, various built-up welding bead application methods have been proposed. One is a bead built up spirally from the edge toward the center of a hole in the base metal to bridge over the hole. Another involves fillet weld reprocessing wherein one makes, seemingly for preventing internal defects from occurring in a fairly large fillet weld, e.g., having a leg wider than an inch, a succession of several ever narrower and narrower beads stacked one over the other and up against a piece of the base metal that makes one part of the fillet joint. (Japanese laid-open Sho-59-50969.) Because the welding energy input of such successive weld layers is cumulative, considerable additional welding energy input can be incurred from such stacking practice at the joint. The energy input can be double or more that of the primary structural weld at the base of the stack even though it is reduced with each ascending layer.
The primary structural welds referred to herein are extended (e.g. at least about 2 inches long, often 6 to 10 inches long and even longer). They are comparatively small, i.e. having maximum length of each fusion face about 20 mm., and often less than half that. They are load-carrying structural elements of weld bead-containing filler metal, and they are used to join pieces of weldable metal. They are to be distinguished from overlay welds or a shape melting practice that is used to build up and/or coat a surface rather than to join pieces of preformed metal.
Temper Bead Technique has been used for tempering the heat-affected zones of structural welds. For example, this technique is used for welding heavy pieces such as high strength steel (HY 80 or 100) for nuclear submarines. There the temper bead or beads lie on the top of the structural weld. Contact between the temper beads and the base metal is to be avoided in most cases. However, when such temper beads are used on single pass and two-pass fillet welds, such otherwise undesirable contact can be tolerated Thus, the toe area of the structural weld and its bordering base metal can be covered with a bead from a temper pass.
Conventional welding practice in laying down a temper bead pass or one or more cover pass beads for a multipass operation characteristically is done while the previous pass still is hot, and it is done at about the same power level (welding energy input) as the primary structural weld passes or the other passes previously made. Normally, also, temper bead passes and such heat treatments extend for the full length of the primary structural weld.
Structurally-welded assemblies, such as those used in truck and other automotive applications, can fail from fatigue. Welded brackets, such as hanger brackets on truck axles, often are fatigue-prone. A fatigue-prone area, section or zone of a weldment typically is a site of stress concentration, e.g. a narrowing, or a sharp change in the structural geometry of the assembly. Sometimes it is just some site that repeatedly shows fatigue cracks for another reason. An example of a fatigue-prone automotive bracket is an air chamber bracket used in the braking system of heavy tractor/trailer rigs. This bracket comprises a steel tube which is normal to and passes through holes in two steel arms; it relies on fillet welds to join the arm and the tube components together and transmit loads. The fillet weld to the arm that has the air cylinder cantilevered from it is representative of a weldment that tends to fail by initiation and propagation of fatigue cracks, typically at a toe on the arm of the fillet weld; such cracks can be followed by substantial fracture.
A small primary structural weld in this application means one which has no fusion face (e.g. a leg of a fillet weld) that is larger than about 20 mm. Typically the welds are about half that size for an air chamber bracket for a heavy-duty truck brake. When reference herein is made to the size of such face or the leg size of a fillet weld, what is meant is a mean average of such face or leg size taken at intervals along the length of the weld.
The usual fillet weld is a weld of approximately triangular cross section joining two surfaces at right angles to each other in a lap joint, T-joint, or corner joint. However, in this application, the intersection of the two surfaces also may form an acute or obtuse angle instead, and the intersection may define a straight or curved line that may be in a single plane or in more than one plane.
Advantages of the present invention over prior proposals include the combination of its being practical for the small primary structural welds as noted above, its economy, its providing the resulting weldments with improved fatigue resistance, and its ease of process control during deposition for obtaining readily a smooth, reasonably uniform overall weld contour with attendant reduction of abrupt shape changes as well as reenforcement of the primary structural weld by the addition of a significant amount of metal to the joint. The invention provides a tempering and a beneficial redistribution of stress in the primary (fillet) structural weld. Significantly, also, there tends to be less joint distortion than if a conventional reprocessing weld were applied in conventional manner over the primary structural root weld, e.g. as a temper bead pass or as a lamination of stacked narrower and narrower beads.
Moreover, use of the invention can act to melt away a weakening notch occurring from a primary structural weld undercut at a toe of the primary structural weld, and to remove slag and nonmetallic inclusions that may be entrapped at a toe of the primary structural weld.
While not intending to be bound by theory as to why the inventive welding method and resulting product are so effective in cutting down on fatigue failures of welded assemblies, it is believed that the combination of stress relieving/tempering at fatiguing portions of the primary structural weld, removing any primary structural weld notch and/or slag or nonmetallic inclusions entrapped at the overlaid part of a toe of the primary structural weld, achieving a smoother (i.e. less abrupt) weld contour there, reenforcing the primary weld, redistributing the stress on the primary weld by assuming some of the stress itself, and low resulting joint distortion from the additional welding energy all contribute to obtaining good resistance of the resulting weld to fatigue cracking.
The specific "welding energy input" used to deposit one or more reprocessing weld beads over a structural weld on a joint root of structural members tends to make the resulting composite weld unique. The magnitude of such input establishes the relative size and scope of the tempering zone and the heataffected zones beneath the composite weld and imparts certain stresses and/or distortions to the resulting weldment; the magnitude of these generally rises with a rise in such input. As the welding energy input of successive welds over a root pass weld is cumulative, the relationship between the magnitude of such input into a root pass weld and the magnitude of the input into the successive reprocessing welds is one practical way to help define one or more reprocessing welds themselves as well as the process of making them. This is because the several concurrent effects that the reprocessing weld can have on the structural weld.
A discovery here which is particularly beneficial is that favorable effects from the welding energy input of a reprocessing weld deposit, such as tempering and strengthening of a welded area, can be effectively preserved to a practical degree without having to encounter and endure appreciable bad effects from the same welding energy input to that reprocessing deposition such as an unsatisfactory degree of weldment distortion or strain. The instant invention involves that discovery.